class User < ActiveRecord::Base
  validates_presence_of :password, :if => :should_validate_password?
  validates_presence_of :state, :on => :create
  validates_presence_of :state, :if => :in_us?

  attr_accessor :updating_password

  def in_us?
    country == 'US'
  end

  def should_validate_password?
    updating_password || new_record?
  end
end

# in controller

@user.updating_password = true
@user.save

@user.save(false)  # will avoid any validation in the model

From railscast #41

Conditional validations

rails, validation

Rails